Why Invest When You're Guaranteed To Lose Money?

People invest because they want to make money. Surely, no-one in their right mind would invest in anything that guarantees they will make a loss? Even in high-risk ventures where the most likely outcome is a loss, there is always the possibility of a big upside. It's like betting on a 20-1 long shot -- you'll probably lose but, if you do win, the payout is a whopping 2,000% return on your stake.